1. Field of the Invention
The rising economic and environmental cost of recovering natural resources has stimulated interest in utilizing methods of recovery to produce as little waste of the resources as possible without substantial increases in cost. Like other enterprises engaged in the extraction of mineral products, mining differs from manufacturing industries in that an increased productivity must inevitably result in a shortened mine life unless the increased productivity results from an improvement in recovery techniques. The invention relates to an improved method of mining trona mineral, such as is found in southwestern Wyoming and similar trona deposits situated elsewhere, with increased recovery of trona over conventional methods without substantial increase in cost.
2. Description of Prior Art
Trona mineral, or natural sodium sesquicarbonate, having the formula EQU Na.sub.2 CO.sub.3 .multidot.NaHCO.sub.3 .multidot.2H.sub.2 O
is presently mined in the United States from trona deposits located in southwestern Wyoming, in the general region of the town of Green River, Wyo. The Wyoming deposits were formed from residues of Gosuite Lake, which covered Wyoming for four million years during the early and middle Eocene age, approximately seventy million years ago. The Eocene age was marked with vast structural depressions and an inland advance of the sea. With increasing dessication over hundreds of thousands of years, the level of Gosuite Lake sank below its outlet to the sea, shrinking until it reached an area of approximately 1000 square miles and its brine content became so concentrated that beds of trona were deposited. Other major trona deposits are located in Kenya at Lake Magadi and in California near Searles Lake and Owens Lake.
The Wyoming trona deposits are evaporites, as hereinabove described, and hence form substantially horizontal layers. The beds vary greatly in thickness, from about 1 foot to about 16 feet. However, the beds are persistent and extend for about 1000 square miles, thus providing reserves adequate for reasonably forseeable future needs. The beds are located approximately 800 feet to approximately 2000 feet below ground surface. In particular, a main trona bed, averaging a thickness of about 8 feet to about 11 feet is located approximately 1200 feet to approximately 1600 feet below ground surface. The main bed is located below substantially horizontal layers of sandstones, siltstones and mainly unconsolidated shales. In particular, within about 400 feet above the main trona bed are layers of mainly weak, laminated green-grey shales and oil shale, interbedded with bands of trona from about 4 feet to about 5 feet thick. Immediately below the main trona bed lie substantially horizontal layers of somewhat plastic oil shale, also interbedded with bands of trona. Both overlying and underlying shale layers contain methane gas.
The comparative compressive strengths, in pounds per square inch, of the main trona bed and of the overlying and underlying shale layers in average values is substantially as follows:
Shale 3,700 p.s.i. PA0 Trona 7,500 p.s.i.
Thus, both the immediately overlying and the immediately underlying layers are substantially weaker than the main trona bed. Recovery of the main trona bed, accordingly, essentially comprises removing the only strong layer within its immediate vicinity.
Presently, it is economical to recover from the Wyoming deposits only the trona located in the main trona bed. This recovery is presently achieved by underground mining. However, because the surrounding shale layers are substantially weaker than the main trona bed and because of the substantial depth at which the main trona bed is located, removal of the main trona bed will leave an essentially weak shale roof which is incapable of supporting itself over large spans. Special techniques have accordingly been devised to mine the trona bed and provide roof support for areas of active mining. No attempt is made to support the entire weight of the overlying materials, roof support in underground mining being generally designed only for the purpose of protecting areas of active mining; i.e., the roof is allowed to cave over mined out areas. These techniques are primarily designed to prevent the caving of the overlying shale layer into an area of active mining and to provide adequate ventilation due to the presence of methane gas in the surrounding shale layers, as hereinabove mentioned, which is released upon caving of the shale layers.
One technique generally used in mining the main trona bed in Wyoming, is the room and pillar system. In this system, trona is generally mined by driving a series of tunnels through the main trona bed in two directions at substantially right angles, so as to divide the bed into a number of blocks or pillars and generating a cellular (rooms) pattern. In particular, essentially rectangular tunnels are cut in the main trona bed, which tunnels are termed secondary entries. The vertical walls of the secondary entries form bases for support of the overburden and its pressure arch. The rectangular shape of the secondary entries, however, provides no support for the span of roof between the substantially vertical walls of the secondary entries. Accordingly, a substantial layer of trona, approximately 1 foot to 4 feet, is left adjacent the roof of the secondary entries. The roof is additionally supported by roof bolting and timbering. Substantially parallel rooms, which are about 15 feet wide and which are spaced about 50 to about 60 feet apart are driven into the main trona bed from the secondary entries, generally using a continuous mining machine. By use of the continuous mining machine, the rooms are elliptically shaped and conform to the pressure arch of the overburden. Accordingly, the roof span between the walls of the rooms is more adequately supported by leaving a layer of trona adjacent the roof of the room than the roof span in the secondary entries. Hence, less secondary support means such as roof bolting and timbering will be required for the rooms. The pillars of trona which remain between the parallel rooms are then extracted by driving tunnels, called lifts, through the pillars. The lifts are separated from the mined out area by a narrow wall or fender of trona, which fender is approximately 7 feet thick or less. When the lift is completed, the fender is removed by blasting and the resulting unsupported roof adjacent the previously mined area is caved. This caving relieves some of the overburden pressure within the pillar being mined. This sequence is repeated and, upon completion of one lift, the fender is blasted and the next parallel lift is begun.
As hereinabove mentioned, the primary disadvantage of any room and pillar system is that a substantial amount of the trona cannot be recovered but must remain to support the overburden. This unrecovered trona is generally left adjacent the roof and in the pillars. Further, as blasting is required to recover the fender, the roof supported by the trona layer and secondarily supported by roof bolting and timbering develops cracks and fissures, and unplanned caving very often results. The greater part of the fender ore is generally not recovered.
Another technique used in mining trona is the longwall mining system. In this system, the whole of the main trona bed within a specified area, called a panel, is extracted in one continuous operation by taking successive slices over the entire length of a long working face. No remnants of trona are left either to support the overburden or to serve as stress concentrators, such as the crushed supporting pillars mentioned hereinabove (while both blasting and crushing occur, they do not necessarily occur together). However, costly roof supporting structures must be used adjacent and parallel to the working face. The length of the trona face will usually be about 300 to about 500 feet, a length of about 300 feet often being considered necessary to achieve an output sufficient to justify the costs of this technique. Further, the machinery used in mining the face cannot be used in development of new mining areas.
In the longwall technique, two parallel tunnels are driven into the main trona bed, starting from the main haulage road of the mine at points approximately 300 feet to 500 feet apart, or however long the working trona face is to be. These parallel tunnels are termed gate roads and they are often driven at right angles to the main haulage road. As the gate roads are ultimately intended for use as transport, they are fairly large. After some distance has been advanced in both gate roads, a longwall trona face is opened up by driving a further tunnel, the height of the bed, between the remote ends of the gate roads. Trona is mined from the exposed longwall by use of either a double drum shearer loader or plow. The shearer or plow is pulled back and forth across the exposed longwall trona face, and loosened trona is dropped into a conveyor. Self-advancing hydraulic jacks or chocks support the roof and follow the plow or shearer as it slices into the trona bed. The hydraulic roof supports, whether chock or shield types, support the immediate roof and overlying layers. When advanced this support is removed and the weight of the overburden creates the fall necessary to remove the excess pressure at the exposed trona face, thereby providing protection from unplanned cavings. Furthermore, the hydraulic roof supports control the caved overburden, or gob, and segregates this caved material from the active working face. The mining can either proceed away from the main haulage road, in which case it would be termed advance longwall mining, or it can proceed towards the main haulage road, in which case it would be termed retreat longwall mining.
In particular, trona panels of about 2,000 feet to about 4,000 feet in length and about 300 feet in width have been successfully mined. In both retreat and advance mining, a waste area is left behind known as goaf or gob. The gob consists generally of the mined-out area in which the roof has collapsed once no longer supported by the self-advancing chocks. However, due to the length of the trona face being mined, it is quite often difficult to induce the initial cave and uneven caving may result, which places excessive weight on all or some of the supports. Further, the plow or shearer used to mine the exposed longwall cannot be used to develop new panels. Hence, the working of the longwall face and development of new panels must be properly spaced or else expensive machinery will be idled.
Another disadvantage associated with longwall mining of trona is cost. As hereinabove described, a typical longwall face is about 300 feet to 500 feet in length, and the practice has been usually to place supports approximately every three to five feet along the exposed trona face. These supports are very costly. Further, this usual length of longwall trona face is generally greater than the pressure arch of the overburden, and accordingly results in a tendency to overload the supports. Additionally, as hereinabove mentioned, the double drum shearer loader or plow cannot be used for development work, and so system development work must be properly paced or major pieces of equipment will be idled.
By the method disclosed herein, the problems associated with such prior art systems as the room and pillar system as well as the longwall mining system are overcome. Thus by the system described herein a more complete and effective recovery from the trona is accomplished. by mining the shorter face of the trona panel, more versatility of equipment results, fewer costly roof supporting structures are needed, and initial and unform caving is easier to achieve than in current practices. Additionally, more trona is recovered than in the predominantly used room and pillar mining of trona.